NEETS Mod 5 - AC Generators

Question 1

What are AC generators generally known as?

Answer 1

Alternators

Question 2

What principle do all electrical generators depend upon?

Answer 2

Magnetic Induction

Question 3

What are the two ways EMF is induced in a coil?

Answer 3

(1) a coil cutting through a magnetic field

(2) a magnetic field cutting through a coil

Question 4

Magnetic Induction occurs when there is relative motion between what two elements?

Answer 4

Conductor and magnetic field

Question 5

What is the part of an alternator in which the output voltage is generated?

Answer 5

Armature

Question 6

What are the two basic types of alternators?

Answer 6

Rotating armature

Rotating field

Question 7

What is the main advantage of the rotating field alternator?

Answer 7

Output voltage is taken directly from the armature (not brushes or slip rings)

Question 8

What are the two types of rotors used in rotating-field alternators?

Answer 8

Turbine-driven

Salient-pole

Question 9

When is a turbine-driven rotor used?

Answer 9

When the prime mover is a high speed turbine.

Question 10

When are salient-pole rotors used?

Answer 10

In low-speed alternators

Question 11

How are alternators usually rated?

Answer 11

By the voltage and maximum current they are designed to produce. (volt amperes)

Question 12

What type of prime mover requires specially designed high-speed alternator?

Answer 12

Steam turbine

Question 13

Salient-pole rotors may be used in alternators driven by what types of prime movers?

Answer 13

Low speed (portable tools & small appliances)

Question 14

What does single phase indicate?

Answer 14

One continuously alternating voltage (output)

Question 15

What is the most common alternator used today?

Answer 15

Three phase

Question 16

What determines the phase relationship between the voltages in a two phase AC generator?

Answer 16

Placement of the armature coils

Question 17

In a three phase alternator, what is the phase relationship between the individual output voltages.

Answer 17

Each phase is displaced 120°

Question 18

What are the two methods of connecting the outputs from a three phase alternator to the load?

Answer 18

Delta & Wye

Question 19

What type of transformers and connections are used to convert 450v, 3 phase power to 117v, single phase?

Answer 19

Single phase, step-down transformer

Question 20

What two factors determine the frequency of the output voltage of an alternator?

Answer 20

Number of poles (NP), multiplied by

Speed of rotation, divided by 120.

Question 21

What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?

Answer 21

4 × 3600 = 14,400

14,400 ÷ 120 = 120

Question 22

The variation in output voltage as the load changes is referred to as what? How is it expressed?

Answer 22

Percentage of regulation

Question 23

How is output voltage controlled in practical alternators?

Answer 23

By varying the voltage applied to the field windings.

Question 24

What generator characteristics must be considered when alternators are synchronized for parallel operation?

Answer 24

Output voltage, frequency, and phase relationships.